Robotic hands are popular end-effectors that have been developed with the aim of matching the human hands in terms of dexterity and adaptation capabilities. Robotic hands are often designed to equip either a dexterous manipulator for pick-and-place tasks or a human being as a prosthetic device. As robots move to new, highly unpredictable environments such as homes or outdoors, the demands placed on their grasping abilities become increasingly complex. Traditional robotic grippers have been split into two broad categories: simple industrial grippers and complex multifingered hands. The former category provides excellent grip strength and simplicity of design but does not allow for dexterous manipulation of grasped objects. The latter category provides agile grasping but suffers from complexity of mechanical design and control.
Traditional dexterous robotic hands, such as the Utah/MIT hand, the Stanford/JPL hand, and the DLR hand, have required large numbers of actuators, leading to elaborate control schemes involving multi-layer computer algorithms and software synergies. In many applications such as prosthetics, such control schemes are impractical or inefficient due to the intensive hardware and software requirements. For instance, weight restrictions for usability of prosthetic hands make fully actuated standalone prosthetic hands very difficult to make with current technology. The need to carry out a wide variety of complicated grasps while maintaining a relatively simple control scheme and low weight has led to the recent development of underactuated fingers that are mechanically intelligent.
Significant efforts have been made to find designs simple enough to be easily built and controlled, and special emphasis has been placed on achieving the required degrees of freedom (DOF) while decreasing the number of required actuators. Some prototypes may be driven by tendons or linkages, which lead to the automatic and mechanical adaptation of the robotic finger to the shape of the object seized. Prominent examples of mechanically underactuated fingers include the SDM hand and the SARAH hand. Such fingers require fewer actuators than the number of degrees of freedom that they possess, relying on mechanical designs incorporating passively compliant elements to allow the hand to respond to the environment and select the best grasp type to perform. More recently, some grippers have been created using the concept of mechanical underactuation to ensure both good versatility and simplicity. These underactuated hands possess many degrees of freedom, allowing for a versatile grasping scheme, but they depend on only a few actuators to realize grasps. Thus, these hands often make use of mechanically compliant elements, particularly springs and limits, to control the grasping process. They can be designed using a wide variety of mechanisms, including tendons, belt drives, and linkages.
For a conventional multi-fingered robotic hand, there are three primary modes of grasping: parallel, coupled, and self-adaptive. Modern hands focus on the combination of these types of grasps. For instance, several parallel and self-adaptive (PASA) grasping hands have already been created using a variety of mechanisms, including coupled and self-adaptive designs using tendons and linkages, and parallel and self-adaptive designs using belt drives. An important property of underactuated fingers is the ability to realize different types of grasps. In particular, a dexterous hand ought to be able to realize both pinch grasps and encompassing grasps. The parallel pinch/grasp is shown in panel (a) of FIG. 1, and the self-adaptive encompassing grasp is shown in panel (b) of FIG. 1. Combining these two grasp schemes leads to a PASA finger. Panels (a) and (b) of FIG. 2 schematically depict two designs of underactuated PASA finger using linkages in the prior art. FIG. 3 demonstrates how the parallel pinch is executed by a prior art linkage design in panel (a), and how the encompassing grasp is executed by a prior art linkage design in panel (b).
However, a problematic issue exists with the current PASA grasping scheme. When the fingers close circularly, they will create a height gap, as shown in FIG. 4. This gap makes it difficult to pick up small objects that rest against a surface using a parallel pinch, a task commonly encountered when picking up items from a table. If the finger starts from the upright position as shown in panel (a) of FIG. 4, it may miss the object entirely as shown in panel (b). If the finger starts beyond the upright position as shown in panel (c) in FIG. 4, it may experience interference with the surface, preventing it from closing as shown in panel (d). In known designs, such a grasp can only be realized if the wrist is moved along with the fingers. This would increase the control complexity, require a highly accurate visual sensor, and sacrifice the benefits of an underactuated finger. Ideally, the fingers can close linearly during the parallel pinch process while still be able to perform a self-adaptive encompassing grasp. In FIG. 5, the gap distance Δs=L1−L1 cos θ, where L1 is the length of the proximal phalanx and θ is its angle deviated from the upright position/orientation/direction. The gap distance Δs also equals to the reduction of the vertical height of the finger. In FIG. 6, panel (a) shows the phalanx positioning during parallel mode, and panel (b) shows the self-adaptive mode. The proximal joint shaft angle θ1 is the angle of the proximal phalanx's orientation deviated from the upright position/orientation/direction (wherein θ1=0). The distal joint shaft angle θ2 is the angle of the distal phalanx's orientation deviated from the proximal phalanx's position/orientation/direction (wherein θ2=0 the distal phalanx and the proximal phalanx constitute a straight line).
People have attempted to design a mechanism that gives the correct compensatory displacement. The most obvious way to accomplish this is to simply translate the rotational motion of the distal joint shaft into a translational motion along the distal phalanx. However, most of simple mechanisms capable of performing such a transformation do so with constrains or less degree of freedom. For example, a change in the angle results in a fixed proportional change in the translational motion, as in a gear rack. While this design may provide a decent approximation of the true gap distance for small angles, the inherent constraints means that such mechanisms do not scale up well and are not suitable for environments where a high degree of precision is required. On the other hand, the desired mechanism should allow varying gap distance varies according to the cosine of the angle. In prior arts, Such a motion may be possible with a large, complicated mechanism or additional actuators however, it remains to be achieved to have a simpler mechanism while maintaining the degree of freedom and mathematical precision in the hand to ensure a robust and reliable motion—that is, truly adaptive grasping.
Advantageously, the present invention provides a novel underactuated finger that is capable of performing adaptive parallel grasping motions without the use of an additional actuator by offering desired compensatory displacement. The compensatory displacement is not fixed by proportional movement by truly adaptive with respect to any environment boundaries. For example, some embodiments of the invention may use an eccentric circular cam revolved about a point on its circumference, lifting a follower whose motion can be amplified to give the precise compensatory displacement needed to ensure a smooth grasping motion. Some embodiments of the present invention solve the problem in the prior art by causing the distal phalanx of the finger to extend and retract/shrink automatically with the motion of the proximal phalanx, and keeping the fingertip's motion linear. Although the embodiment depicted herein shows two phalanges, a person skilled in the art will realize that the same principle can be applied to fingers having any number of phalanges.